In general, a filter has been used for filtration and purification of a fluid having a low concentration of a solids content and high flowability in the fluid containing powder fine particles.
One indication of filter performance is a filtration life representing duration during which the filter can be used and a cumulative flow. As an attempt for improving the filtration life, for example, a material prepared by providing at least two layers of a prefiltration layer and a microfiltration layer within a filter composed of a nonwoven fiber aggregate, in which the prefiltration layer is configured to be thinned in a fiber diameter of constituent fibers in a filtration direction, and the microfiltration layer is composed of the nonwoven fiber aggregate containing fibers finer than a minimum fiber diameter in the prefiltration layer (Patent literature No. 1).
However, in order to configure the prefiltration layer to be thinned in the fiber diameter in the filtration direction, the nonwoven fiber aggregate in which a mean fiber diameter is changed in a length direction has been required to be arranged in advance. In order to produce such a nonwoven fiber aggregate, special spinning conditions should be set up, and therefore such production is disadvantageous in terms of cost. Moreover, the nonwoven fiber aggregate that can be produced has a limit in the minimum fiber diameter, and therefore such production has not been necessarily sufficient also in view of microfiltration performance.
Moreover, in order to obtain high precision filter having satisfactory form retention properties and an excellent balance between filter filtration precision and the filtration life, a proposal has been made on a nonwoven fabric for a filter, in which at least two layers of nonwoven fabrics are laminated, and a packing factor of the nonwoven fabric on an upper layer side is adjusted to 0.3 to 0.8, and a packing factor of the nonwoven fabric on a lower layer side is adjusted to 0.01 to 0.25 (Patent literature No. 2). In the relevant invention, a micro space is retained on a contact surface between a nonwoven fabric layer and a support material by arranging the nonwoven fabric having a low packing factor on the lower layer side of the filter, and utilization efficiency and the form retention properties of the filter are improved by acting the layer also as a cushioning material.
However, a filtration material to be targeted in the literature described above is an aqueous solution in which 0.6 μm alumina particles are dispersed thereinto with a concentration of 50 ppm, more specifically, the art is to filtrate a liquid containing the particles having a significantly small particle diameter and a low concentration.
Meanwhile, in order to shorten a drying time after filtration, to achieve volume reduction of the amount of condensation of a volatilized liquid, or the like, achievement of high concentration of the solids content has been required.
If the solids content is highly concentrated, the fluid results in poor flowability, and interaction of powders being the solids content to each other is strengthened. Therefore, in the case of a conventional cartridge filter for water, a problem has been known in which a rash phenomenon occurs when powder particles pass through the filter even with the powders having a diameter smaller than a mean pore diameter of the filter, powder aggregation (bridge) is caused, and an apparent particle diameter thereof is increased to cause clogging of the filter.
Patent literature No. 3 proposes a filter in which a problem of change in filtration precision by a differential pressure and shortening of a filter life in filtration of a viscous fluid is solved, and simultaneously even if a pulse pressure or a high differential pressure is caused, a soft gel-form solid can be caught. The invention of Patent literature No. 3 relates to an art in which a filter has, in a main filtration layer of the filter, a first main filtration layer subjected to compression bonding treatment to have a void ratio of 50 to 80%, and a second main filtration layer without being subjected to compression bonding treatment to have a void ratio of 80% or more, and a nonwoven fabric in the second main filtration layer is configured to be 1.2 times or more a nonwoven fabric in the first main filtration layer.